This invention relates to a new ceramic particularly, but not exclusively, useful as a cathode in a solid oxide fuel cell (SOFC). A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel and an oxidizing gas via an ion conducting electrolyte. The chief characteristic of a fuel cell is its ability to convert chemical energy directly into electrical energy without the need for combustion, giving much higher conversion efficiencies than conventional thermo-mechanical methods (e.g. steam turbines). Consequently fuel cells have much lower carbon dioxide emissions than fossil fuel-based technologies for the same power output. They also produce negligible amounts of SOx and NOx, the main constituents of acid rain and photochemical smog.
(SOFCs) are constructed entirely from solid-state materials; they utilize a fast oxygen ion conducting ceramic as the electrolyte, and operate in the temperature range 900–1000° C. SOFCs provide the following advantages compared with other fuel cell types:                few problems with electrolyte management (cf liquid electrolytes, which are typically corrosive and difficult to handle);        highest efficiencies of all fuel cells (50–60%);        high-grade waste heat is produced, for combined heat and power (CHP) applications;        internal reforming of hydrocarbon fuels (to produce hydrogen and methane) is possible.        
A single SOFC unit consists of two electrodes (an anode and cathode) separated by the electrolyte, see FIG. 1. Fuel (usually hydrogen, H2, or methane, CH4) arrives at the anode, where it reacts with oxygen ions from the electrolyte, thereby releasing electrons (e−) to the external circuit. On the other side of the fuel cell, oxidant (e.g. O2 or air) is fed to the cathode, where it supplies the oxygen ions (O2−) for the electrolyte by accepting electrons from the external circuit. The electrolyte conducts these ions between the electrodes, maintaining overall electrical charge balance. The flow of electrons in the external circuit provides useful power.
Today's technology employs several ceramic materials as the active SOFC components. The anode is typically formed from an electronically conducting nickel/yttria-stabilized zirconia (Ni/YSZ) cermet (i.e. a ceramic/metal composite). The cathode is based on a mixed conducting perovskite, lanthanum manganate (LaMnO3) being the preferred prior art material. Yttria-stabilized zirconia (YSZ) is preferably used for the oxygen ion conducting electrolyte. To generate a reasonable voltage, fuel cells are not operated as single units but as an array of units or “stack”, with a doped lanthanum chromite (e.g. La0.8Ca0.2CrO3) interconnect joining the anodes and cathodes of adjacent units. The most common configuration is the planar (or “flat-plate”) SOFC illustrated in FIG. 1.
Efforts to lower the cost of the solid oxide fuel cell (SOFC) have driven the operating temperature of the conventional SOFCs down from 1000° C. to 800° C. and lower, where less expensive and more practical metal components can be used, such as in the bipolar plate and gas manifolding. However, the lower operating temperature increases the ohmic and polarization resistances within the cell resulting in unacceptable performance losses for presently used materials. The polarization losses arise from reactions at both the anode and cathode, of which the cathode contribution is the more significant one. The conventional cathode material is strontium-doped lanthanum manganite (LSM), which is predominantly an electronic conductor with negligible oxygen ion conductivity. LSM is thermally and chemically compatible with the YSZ electrolyte and has adequate electrochemical performance at 1000° C. However, below 900° C., the polarization loss in LSM becomes too large for effective operation. It is thought that the lack of oxygen ion conductivity severely limits the performance of LSM at lower temperatures. Many studies have focused on alternative cathode materials, the most promising of which have been found in the perovskite (ABO3) family. Most of the better performing cathodes contain lanthanum and strontium on the A-site, with mixtures of Co, Fe, Ni, Cu, Cr, and Mn on the B-site, for instance, see the Christensen et al. U.S. Pat. No. 6,150,290. With the exception of Cr, reaction products between the cathode and electrolyte have been observed for the remaining B-site constituents, when in contact with YSZ at temperatures of 1000° C. and above. At 800° C., Mn and Fe also show no reaction products.
Iron and cobalt-based perovskite cathodes have oxygen ion conductivities several orders of magnitude higher than LSM, which dramatically increases the reaction area for adsorption and incorporation of oxygen on the cathode. This improved mixed conductivity facilitates a greater flux of oxygen ion incorporation into the electrolyte and thus a higher current.
Previously, the ferrites (La(Sr)FeO3-LSF) as cathodes on YSZ have not been described in detail. Iron has, more typically, been a dopant on the B-site of cobaltite and manganite cathodes. With the recent search for lower temperature cathodes, the ferrites have re-emerged as potential materials. Certain LSF compositions (typically when Sr-doping levels are between 20–30 mol %) show only minimal reactivity towards YSZ and relatively close thermal expansion matching. However, stoichiometric LSF does not achieve a low enough polarization resistance at 800° C. to be a useful cathode. Doping on the B-site has proved successful when using Ni and Co but reactivity with YSZ was observed even at the 20 mol % doping level, which degraded the cathode performance dramatically.
Common for the manganite perovskites is the use of a small 1–5 mol % A-site deficiency to prevent reactivity of the manganite with YSZ. By using an A-site deficient LSM cathode, the precipitation of La2O3 and SrO are dramatically reduced and very little La2Zr2O7 or SrZrO3 reaction product has been observed in these cases. Very limited studies have been performed on the cobalt-based perovskites regarding A-site deficiency suggesting that A-site deficiencies of up to 10 mol % do not prevent the reaction between the cobaltites and YSZ electrolyte. A recent study into LSF has reported similar effects to that of LSM. It is postulated that lanthanide and A-site deficiencies are compensated in the ferrite and cobaltite systems by the introduction of oxygen vacancies and some electronic defects.
A few papers discuss the effect of A-site deficiency on the electrochemical and electrocatalytic performance of manganese-based perovskite cathodes. X-ray diffraction results obtained by various groups are in close agreement and identify the limit of A-site deficiency before secondary phases start to appear at 10 mol % for La1-xMnO3 and progressively lower values as the Sr-doping level increases. The optimum conductivity and cathode performance for LSM have been obtained with an A-site deficiency around 5 mol %. At this relatively low A-site deficiency it is believed that the diminished reactivity toward YSZ results in the improved cathode performance and only a marginal effect derives from the improved electrochemistry.